Why It Hurts. Dr. Aneesh Singla

      Two important aspects contributing to peripheral sensitization are pain and inflammation. In particular, the two effects feed into each other, which I believe forms the circular feedback loop that drives this phenomenon. If the injury heals, then things return to normal in most cases. But in some instances, persistent pain causes wind-up, leading to central sensitization, which is how the body adjusts to the perception of ongoing pain. (For more on wind-up and central sensitization, see Chapter 1.) If your body is warning you of an ongoing injury, it will continuously sound the alarm, whether or not it is warranted. If your body senses that you will ignore the alarm, it will lower the threshold for the alarm, so with time, even the slightest pressure will elicit a painful signal. In effect, the pain alarm will become more sensitive, and you will become more sensitized to certain movements or stimuli.

      To help explain wind-up and central sensitization, I will use an interesting case reported by D. W. Wheeler, et al., regarding a female patient with CIP. She became pregnant and, during childbirth, sustained multiple pelvic fractures. This meant that a C-section became necessary to prevent injury to herself or her baby. Due to injuries sustained during this fraught delivery, including injury to a nerve root in her spine, she began to experience pain for the first time in her life. Skin testing was used to determine her thresholds of sensitivity. This revealed that she was 10 times more sensitive to sensations than she had been prior to her pregnancy. Wind-up had dramatically restored the patient’s ability to feel pain.

      Clearly, the nervous system is adaptive and can change. We call this trait neuroplasticity. Consider that wind-up was able to restore pain to someone previously incapable of feeling it. This should give you some sense of how excruciating wind-up can be for a person experiencing a ten-fold (or more) amplification of normal sensitivity to pain.

      Inflammation can be driven by pain receptors in the skin. Pain receptors can drive the body’s inflammatory response, directly resulting in itching and discomfort. In a recent study published in Nature, scientists shut down the pain receptors in the skin of one group of mice. As a result, the mice had a lowered immune response.

      This discovery fits with the observations I have made about my own patients. For example, after surgery patients often experience a stress response, similar to peripheral sensitization, with an increased level of inflammation. When we aggressively treat pain postoperatively, there is less inflammation and, in my belief, better postoperative recovery. My surgical colleagues at Mass General would routinely observe that the patients with epidurals for postoperative pain seemed to fare better after surgery, perhaps due to attenuation of the stress response and less inflammation.

      Pain and inflammation are like a healthy marriage; when working well together, they are acting synergistically to maintain and protect us from outside invasion or threats. When there is an imbalance or lack of coordination, both can escalate until a pathological pain state occurs.

      To interrupt the feedback loop, I treat patients with a dual approach: First, I administer steroids, which are anti-inflammatories. Then I use local anesthetics to numb the pain. This treatment approach breaks the feedback cycle at two points, more effectively than using either agent alone.

      If we learn from adaptive pain, then it can serve a valuable purpose. But if we let pain spin out of control, it can become maladaptive. In that case, the feedback loop conditions a person to feel the pain in the absence of a physical cause. This poses the question: Can maladaptive pain be un-learned?

      If you know that the problem generating the pain is not life threatening, you may have to simply adjust your routine around the pain, just as you adjusted your stride mid-way in your marathon. But when your body continues to sound the alarm and doesn’t respond completely to an anti-inflammatory or local anesthetic, you have to be careful not to ignore that pestering warning signal. If we fail to shut the pain alarm off with conservative therapy, it is possible that our body is desperately trying to indicate that there is a serious issue at hand (as is the case with appendicitis).

      And so, once more we have discovered that the body has a way of fighting back to keep sending you the pain alarm when it senses something is wrong. When these rare, but serious events occur in our lives, I believe that we actually want to let our bodies continue to sound the alarm (i.e., continue to send the warning signal of pain). If we switch off the pain completely, through over-medication, we risk missing the serious problem that is occurring. Here again, I have observed that over-medicating patients rarely results in 100 percent pain relief. Again, this is the body’s natural intelligence preserving our ability to feel pain in the setting of a major systemic problem.

      The Body’s Protective Warning

      One patient, David, saw me for pain control after spine surgery. He was managing his pain with high doses of opiates but he was still clearly uncomfortable.

      “Is this how I am going to be for the rest of my life?” he asked. “I was expecting to be fixed by now.”

      I simply couldn’t find an effective way to control his pain, so David went back to see his surgeon, who subsequently diagnosed an infection in David’s spine. Another surgery removed the new infection and after that the pain completely resolved. If I’d simply turned off his pain at his request, that infection would have spread.

      Another similar case comes to mind. A friend, Nancy, was training for a marathon when she developed severe hip pain. At first, she thought it was a strain. She kept running under the assumption that the pain was muscular and that the best thing to do was just push through it. Unfortunately, it didn’t get better.

      Nancy’s X-rays came back normal so an MRI was ordered, and that revealed a stress fracture. Nancy had to pull out of training and spend four weeks on crutches. Without that diagnosis, she might have run the marathon with painkillers, causing a more severe injury, perhaps even a displaced hip fracture.

      A spine surgeon colleague once referred a patient with a disc herniation to me, requesting a nerve root block. This is a procedure where a local anesthetic and a steroid are delivered directly to the patient’s spine to temporarily decrease the perception of pain and decrease the inflammation along the nerve.

      We use nerve root blocks when we suspect inflammation or compression of the spinal nerve by a herniated disc. The nerve block serves diagnostic and therapeutic purposes: If the patient sees a significant benefit, the surgeon can perform highly localized surgery to free the compressed nerve and alleviate the pain with the smallest possible intervention. In other cases there may be multiple compressed nerves, in which case the nerve root block will not provide the same complete relief.

      In this case, we performed the nerve block and the patient did well, finding complete relief while the local anesthetic was working. This established the diagnosis and I planned to send her back to the spine surgeon for a procedure to permanently solve the problem. Given the severe compression, we were both certain she would require surgery.

      The funny thing was, her pain never returned. This experience taught me that, on occasion, simply abolishing the pain temporarily can have a lasting effect. Perhaps the injection interrupts pain just long enough to stop the wind-up phenomenon from spiraling out of control. Perhaps the nerve pathway simply stops sending off the alarm that something is wrong. Perhaps the finely tuned inflammatory response receives a signal to turn off for a while because the communication with the sensory nerves is turned off. This is sometimes the case when there is no serious problem present, but simply nerve irritation, which once treated results in extended pain relief. The reprieve from pain can help turn off the alarm, as long as there is no serious problem lingering. It also helps us as physicians differentiate what is a mechanical compression problem of the nerve versus simple inflammation. The spine surgeon who referred me the patient was somewhat perplexed as well. He wondered whether I’d used steroids in the injection, but I hadn’t.